FIGS. 7(a) and 7(b) show a known semiconductor laser device of a loss guide type having a current injection region in the neighborhood of the laser facet. FIG. 7(a) shows transverse cross-sectional view the neighborhood of the laser facet and the inside of the the cavity. FIG. 7(b) shows a longitudinal cross-sectional view of the laser and particularly the oscillation region 8.
In FIGS. 7(a) and 7(b), reference numeral 1 designates an n type GaAs substrate. A first cladding layer 2 comprising n type Al.sub.0.5 Ga.sub.0.5 As is disposed on the GaAs substrate 1. A p type Al.sub.0.15 Ga.sub.0.85 As active layer 4 is disposed on the first cladding layer 2. A p type second cladding layer 5 comprising p type Al.sub.0.5 Ga.sub.0.5 As having a stripe shaped ridge portion except in the neighborhood of the cavity facets is disposed on the active layer 4. An n type GaAs current blocking layer 6 is disposed on the second cladding layer 5. A p type GaAs contact layer 7 is disposed on the n type GaAs current blocking layer 6. Reference numeral 8 designates a laser oscillation region and reference numeral 9 designates a current non-injection region.
The device will operate as follows.
In the laser oscillation region 8, minority carriers in a concentration of of about 10.sup.18 per cm.sup.3 are injected into the active layer 4, producing a population inversion and a large portion of the injected carriers are converted into photons. In the current non-injection region 9, because the quantity of minority carriers injected is small, excitation minority carriers by the laser light increases. The density of the excited carriers is significantly smaller than the density of injected carriers. By this effect, the non-radiative recombination of electron hole pairs via the surface energy levels at the facet of the active layer are largely reduced relative to a case where there is no current non-injection region 9. Therefore, the light output level at which the destruction of cavity facet due to light increases, that is, the so-called COD level is increased.
In the semiconductor laser device of FIGS. 7(a) and 7(b) however, in order to confine the light in the transverse direction through reflection by the blocking layer into the cavity, the thickness of the second cladding layer 5, except at the stripe ridge, is about 0.3 micron. Therefore, the thickness of the second cladding layer 5 in the neighborhood of the resonator facet is also about 0.3 micron. Because the current blocking layer 6 is GaAs, the laser light is absorbed, and the light absorption coefficient is extremely high. Furthermore, as shown in FIG. 8, the light emission region is broadened inside the laser to the same extent as the second cladding layer including the active layer as its center. In the neighborhood of the cavity, however, because there exists a current blocking layer having a high light absorption coefficient quite close to the active layer, the light intensity does achieve the distribution shown in FIG. 8, but becomes unstable. Accordingly, no optical mode can be obtained. Even if an optical mode is established stable, it is not at the resonator facet. Thus, it is not possible to obtain a stable optical characteristic.
Furthermore, when the light output is to be increased, the absorption of light by the active layer may melt the active layer. Thus, there is a limit to the increase in the of COD level that can be achieved even if a cavity facet non-injection structure is employed.